Palladium has been long used as a reagent for organic syntheses. For example, the Wacker process capable of producing acetaldehyde from ethylene by using palladium chloride as a catalyst has been widely industrialized. On the other hand, the low-valent complexes of palladium are drawing attention of the industry for their availability as catalysts for organic syntheses such as dimerization of 1,3-dienes, and for this purpose, there are known a variety of zero-valent palladium complexes containing an organic phosphine as the ligand. These complexes have structures which differ in coordination number depending on the type of the ligand used, and naturally they differ from each other in physicochemical properties as well as in catalytic performance. However, the known zero-valence palladium phosphine complexes are unstable because they are low in heat resistance and also slowly decompose when left in the air. Although some types of such complexes have a relatively high decomposition temperature, they are vulnerable to oxidation in the air. Therefore, a keen need has developed in the industry for a zero-valent palladium complex which is stable in the air and yet high in heat resistance and is able to serve as an excellent catalyst for the organic syntheses.
There are known various methods for effecting dimerization of 1,3-dienes, including a method in which a combination of a divalent palladium salt and phosphine is used by reducing it to a lower state of valency in the reaction system and a method using a zero-valent palladium complex where an organic phosphine is incorporated as the ligand. The former method, however, is poor in efficiency as it is subject to restrictions in the reaction conditions such as dimerization temperature due to the essential requirement for reduction. Also, since there exist active materials of diverse valencies in the reaction system, the proportion of formation of the object reaction product (hereinafter referred to as selectivity) is low. As regards the latter method, although a variety of palladium complexes are known, each of them is a zero-valent palladium complex with a coordination number greater than 3. The complexes with a high coordination number are weak in activity and hence low in reaction rate in actual use. Thus, strong need has been felt for a catalyst which is further improved in selectivity, strong in activity even at low temperatures and high in reaction rate.
When a palladium-phosphine complex is used as a catalyst for an organic synthesis, the reaction generally proceeds in a homogeneous system. Therefore, this type of complex is very useful in performing the organic syntheses. However, because of the expensiveness of such complexes, it is highly desirable in the industrial processes to efficiently recover the complex after its use for putting the recovered complex to reuse. Since the palladium complex remains dissolved in the reaction solution, it has been generally practice to put the residual reaction solution to reuse or to recover metallic palladium by precipitating it from the residual solution. The former method, however, encounters the problem of an increasing amount of residual solution due to byproduction of high-boiling matter in repeated use of the solution and also the problem of treatment of that portion of the catalyst which has lost its activity during the reaction. On the other hand, the latter method encounters the problem of a complicated operation for reactivating the recovered material, and also this method, although capable of recovering palladium, involves difficulties in recovery of organic phosphine.